package com.example.csdn.view.Text;

import android.content.Context;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.VelocityTracker;
import android.view.ViewConfiguration;
import android.view.animation.DecelerateInterpolator;
import android.widget.Scroller;
import android.widget.TextView;
/**
 * 配置android:scrollbars="vertical"，启用垂直滚动条
 * 
 * 实现水平滚动就是修改mScrollX，可参考HorizontalScrollView
 *
 * 滚动原理： 在绘制前对画布进行偏移操作
 * 
 * 下面是View的绘制机制：
 * |- view.computeScroll()  --用来对mScrollX/Y进行修改。由于在绘制前调用，可调用invalite()来触发
 * |- canvas.translate(-mScrollX,-mScrollY)  --偏移画布
 * |- view.draw()  --绘制
 *  
 * 上述内容可以在View.buildDrawingCache()或ViewGroup.dispatchDraw()->drawChild()中找到.直接查看方法名即可
 * 
 * 滚动帮助类：
 * Scroller --用来计算滚动后的偏移值.具体请参考ScrollView和HorizontalScrollView
 * VelocityTracker --速度计算类。根据fling时的按下、抬起动作，计算滚动初速度
 * 
 * ScrollTextView--流程解析：
 * 1、onTouchEvent() --使用Scroller来计算滚动偏移值
 * 2、重写computeScroll() --对View的mScrollY进行修改， 此处控制滚动范围
 *
 * 滚动范围：
 * 最小值：0
 * 最大值：所有文本高度+内边距-View高度。也就是超出屏幕的文本高度
 */
public class ScrollTextView extends TextView {
    private Scroller mScroller;
    private int mTouchSlop;
    private int mMinimumVelocity;
    private int mMaximumVelocity;
    
    private float mLastMotionY;
    private boolean mIsBeingDragged;
    private VelocityTracker mVelocityTracker;
    private int mActivePointerId = INVALID_POINTER;
    
    
    private static final int INVALID_POINTER = -1;

    public ScrollTextView(Context context, AttributeSet attrs, int defStyle) {
    super(context, attrs, defStyle);
    initView();
    }

    public ScrollTextView(Context context, AttributeSet attrs) {
    super(context, attrs);
    initView();
    }

    public ScrollTextView(Context context) {
    super(context);
    initView();
    }

    private void initView() {
    final Context cx = getContext(); 
    //设置滚动减速器，在fling中会用到
    mScroller = new Scroller(cx,new DecelerateInterpolator(0.5f));
    final ViewConfiguration configuration = ViewConfiguration.get(cx);
        mTouchSlop = configuration.getScaledTouchSlop();
        mMinimumVelocity = configuration.getScaledMinimumFlingVelocity();
        mMaximumVelocity = configuration.getScaledMaximumFlingVelocity();
        
    }
    
    /**
     * 此方法为最后机会来修改mScrollX,mScrollY.
     * 这方法后将根据mScrollX,mScrollY来偏移Canvas已实现内容滚动
     */
    @Override
    public void computeScroll() { 
    super.computeScroll();
        
    final Scroller scroller = mScroller;
    if(scroller.computeScrollOffset()) { //正在滚动，让view滚动到当前位置
        int scrollY = scroller.getCurrY();
        final int maxY = (getLineCount() * getLineHeight() + getPaddingTop() + getPaddingBottom()) - getHeight();
        boolean toEdge = scrollY < 0 || scrollY > maxY;
        if(scrollY < 0) 
            scrollY = 0;
        else if(scrollY > maxY)
            scrollY = maxY;
            
        /*
         *下面等同于：
         * mScrollY = scrollY;
         * awakenScrollBars(); //显示滚动条，必须在xml中配置。 
         * postInvalidate(); 
         */
        scrollTo(0, scrollY);
        if(toEdge) //移到两端，由于位置没有发生变化，导致滚动条不显示
            awakenScrollBars();
        }
    }
    
    public void fling(int velocityY) {
    final int maxY = (getLineCount() * getLineHeight() + getPaddingTop() + getPaddingBottom()) - getHeight();

        mScroller.fling(getScrollX(), getScrollY(), 0, velocityY, 0, 0, 0, 
                Math.max(0, maxY));

        //刷新，让父控件调用computeScroll()
        invalidate();
    }
    
    
    @Override
    public boolean onTouchEvent(MotionEvent ev) {
    /*
     * 事件处理方式：先自己处理后交给父类处理。
     * PS:方式不同，可能导致效果不同。请根据需求自行修改。
     */
    boolean handled = false;
    final int contentHeight = getLineCount() * getLineHeight();
    if(contentHeight > getHeight()) {
        handled = processScroll(ev);  
    }
        
    return handled | super.onTouchEvent(ev);
    }

    private boolean processScroll(MotionEvent ev) {
    boolean handled = false;
    if (mVelocityTracker == null) {
            mVelocityTracker = VelocityTracker.obtain();
        }
        mVelocityTracker.addMovement(ev); //帮助类，用来在fling时计算移动初速度
        
    final int action = ev.getAction();
        
    switch (action) {
    case MotionEvent.ACTION_DOWN: {
        if(!mScroller.isFinished()) {
        mScroller.forceFinished(true);
        }
            
        mLastMotionY = ev.getY();
            mActivePointerId = ev.getPointerId(0);
            mIsBeingDragged = true;
            handled = true;
        break;
    }
    case MotionEvent.ACTION_MOVE: {
        final int pointerId = mActivePointerId;
        if(mIsBeingDragged && INVALID_POINTER != pointerId) {
        final int pointerIndex = ev.findPointerIndex(pointerId);
        final float y = ev.getY(pointerIndex);
        int deltaY = (int) (mLastMotionY - y);
                
        if(Math.abs(deltaY) > mTouchSlop) { //移动距离(正负代表方向)必须大于ViewConfiguration设置的默认值
            mLastMotionY = y;
                    
            /*
             * 默认滚动时间为250ms，建议立即滚动，否则滚动效果不明显
             * 或者直接使用scrollBy(0, deltaY);
             */
            mScroller.startScroll(getScrollX(), getScrollY(), 0, deltaY, 0); 
            invalidate(); 
            handled = true;
        }
        }
        break;
    }
    case MotionEvent.ACTION_UP: {
        final int pointerId = mActivePointerId;
        if(mIsBeingDragged && INVALID_POINTER != pointerId) {
        final VelocityTracker velocityTracker = mVelocityTracker;
        velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
        int initialVelocity = (int) velocityTracker.getYVelocity(pointerId);
                
        if(Math.abs(initialVelocity) > mMinimumVelocity) {
            fling(-initialVelocity);
        }
                
        mActivePointerId = INVALID_POINTER;
                mIsBeingDragged = false;

                if (mVelocityTracker != null) {
                    mVelocityTracker.recycle();
                    mVelocityTracker = null;
                }
                
                handled = true;
       }
       break;
    }
    }
    return handled;
   }    
}